The present invention relates to a telemetric process and apparatus. It more particularly applies to detection systems in the field of mobile robotics.
In order to be effective, certain types of mobile robots must be able to evaluate their environment over distances between 0.5 and 10 m. For this purpose the robots are equipped with one or more telemeters, which can make it possible to reconstitute an image of the environment.
Several telemeter types are known. A first type performs a triangulation in order to determine the distance from a reference point to a point on an object. The triangulation consists of measuring the angle formed by the direction of a continuous light beam and the direction under which the impact point of the beam on the object is seen by a detector. When carried out in this way, the distance measurement is precise, but suffers from a major disadvantage, namely that it only has a limited depth of field.
A second known telemeter type carries out phase shift measurements between a modulated light beam emitted towards a target and a beam reflected by said target. The knowledge of the phase shift makes it possible to determine the distance separating the telemeter from the target.
A major disadvantage of such a telemeter is the detection of parasitic reflections from indirect paths of the beam. Thus, as a result of the large aperture of the detector, it can collect beams having undergone multiple reflections on the target. In addition, the phase shift measured between these parasitic beams and the incident beam leads to erroneous information concerning the distance to be determined.
A third known telemeter type, called a flight time telemeter, uses the measurement of the outward and return time of a light pulse between the telemeter and the target for determining the distance separating them, which is possible because obviously the speed of light is known.
This type of telemeter obviates the multiple reflection and diffusion phenomena. Thus, the determination of the transit time is terminated during the first detection of a reflected light pulse (result of the shortest path) and the apparatus takes no account of any subsequently arriving parasitic pulse. Such a telemeter leads to accuracies of a few centimeters over a distance of about 10 meters. This accuracy can be further improved by determining the mean of a large number of measurements concerning the same impact point. However, this improvement is obtained to the detriment of the calculation time.
A first problem of flight time telemeters is caused by the very significant dynamics (which can reach 10.sup.7) of the light signals detected following a reflection on the target. Thus, the intensity of these signals is very intimately linked with the nature of the target. The reflection coefficient of the latter can change very suddenly from one point to another.
The electronic systems of the telemeter must be able to adapt to amplitude dispersions in order to retain a good accuracy on the light pulse path time measurement. For this purpose, automatic logarithmic amplification or gain checking devices are known. However, apart from the difficulty of constructing these devices, the fluctuation of their propagation time as a function of the dynamics of the detected signals makes them unusable in telemetry.
In flight time telemetry, a chronometer is activated simultaneously with the emission of a light pulse. This chronometer is stopped when a reflected pulse is detected. In order to stop the chronometer, use is presently made of a comparator, which emits a stop signal when it receives an electric signal corresponding to a light signal above a given threshold. However, such a comparator has a time spread on the emission time of the stop signal, as a function of the characteristics of the detection (dynamics, signal-to-noise ratio), which reduces the accuracy of the measurement.